Download Finding the Equivalence Point (calculation method)

Finding the Equivalence Point
(calculation method)
• Strong Acid vs. Strong Base
– 100 % ionized! pH = 7 No equilibrium!
• Weak Acid vs. Strong Base
– Acid is neutralized; Need Kb for conjugate base
equilibrium
• Strong Acid vs. Weak Base
– Base is neutralized; Need Ka for conjugate acid
equilibrium
• Weak Acid vs. Weak Base
– Depends on the strength of both; could be conjugate
acid, conjugate base, or pH 7
Exactly 100 mL of 0.10 M HNO2 are titrated with 100 mL of
a 0.10 M NaOH solution. What is the pH at the
equivalence point ?
start (moles)
0.01
0.01
HNO2 (aq) + OH- (aq)
NO2- (aq) + H2O (l)
end (moles)
0.0
0.0
0.01
Final volume = 200 mL
NO2- (aq) + H2O (l)
Initial (M)
Change (M)
0.01
= 0.05 M
0.200
OH- (aq) + HNO2 (aq)
[NO2-] =
0.05
0.00
0.00
-x
+x
+x
x
x
Equilibrium (M) 0.05 - x
[OH-][HNO2]
x2
-11
=
2.2
x
10
Kb =
=
[NO2-]
0.05-x
pOH = 5.98
0.05 – x  0.05 x  1.05 x 10-6 = [OH-]
pH = 14 – pOH = 8.02
Complex Ion Equilibria and Solubility
A complex ion is an ion containing a central metal cation
bonded to one or more molecules or ions.
Co2+ (aq) + 4Cl- (aq)
Co(H2O)2+
6
CoCl42- (aq)
CoCl24
16.10
16.10
Complex Ion Formation
• These are usually formed from a transition
metal surrounded by ligands (polar
molecules or negative ions).
• As a "rule of thumb" you place twice the
number of ligands around an ion as the
charge on the ion... example: the dark blue
Cu(NH3)42+ (ammonia is used as a test for
Cu2+ ions), and Ag(NH3)2+.
• Memorize the common ligands.
Common Ligands
Ligands
Names used in the ion
H2O
NH3
aqua
ammine
OHClBrCNSCN-
hydroxy
chloro
bromo
cyano
thiocyanato (bonded through
sulphur)
isothiocyanato (bonded through
nitrogen)
Names
• Names: ligand first, then cation
Examples:
– tetraamminecopper(II) ion: Cu(NH3)42+
– diamminesilver(I) ion: Ag(NH3)2+.
– tetrahydroxyzinc(II) ion: Zn(OH)4 2-
• The charge is the sum of the parts
(2+) + 4(-1)= -2.
When Complexes Form
• Aluminum also forms complex ions as do some post
transitions metals. Ex: Al(H2O)63+
• Transitional metals, such as Iron, Zinc and Chromium,
can form complex ions.
• The odd complex ion, FeSCN2+, shows up once in a
while
• Acid-base reactions may change NH3 into NH4+ (or vice
versa) which will alter its ability to act as a ligand.
• Visually, a precipitate may go back into solution as a
complex ion is formed. For example, Cu2+ + a little
NH4OH will form the light blue precipitate, Cu(OH)2. With
excess ammonia, the complex, Cu(NH3)42+, forms.
• Keywords such as "excess" and "concentrated" of any
solution may indicate complex ions. AgNO3 + HCl forms
the white precipitate, AgCl. With excess, concentrated
HCl, the complex ion, AgCl2-, forms and the solution
clears.
Coordination Number
• Total number of bonds from the ligands to
the metal atom.
• Coordination numbers generally range
between 2 and 12, with 4 (tetracoordinate)
and 6 (hexacoordinate) being the most
common.
Some Coordination Complexes
molecular
formula
Lewis Lewis acid
base/ligand
donor coordination
atom
number
Ag(NH3)2+
NH3
Ag+
N
2
[Zn(CN)4]2-
CN-
Zn2+
C
4
[Ni(CN)4]2-
CN-
Ni2+
C
4
[PtCl6] 2-
Cl-
Pt4+
Cl
6
[Ni(NH3)6]2+
NH3
Ni2+
N
6